Organic electroluminescent (EL) devices, or organic light-emitting diodes (OLEDs), are electronic devices that emit light in response to an applied potential. The structure of an OLED includes, in sequence, an anode, an organic EL unit, and a cathode. The organic EL unit disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al., “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987), and commonly assigned U.S. Pat. No. 4,769,292 demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly includes a host material doped with a guest material wherein the layer structures are denoted as HTL/LEL/ETL. Further, there are other multilayer OLEDs that contain more functional layers in the devices. At the same time, many kinds of EL materials are also synthesized and used in OLEDs. These new structures and new materials have further resulted in improved device performance.
In recent years, EL devices have expanded to include not only single color emitting devices, such as red, green and blue, but also devices that emit white light. Monochrome, multi-color and full color display devices can be prepared. The precision shadow mask pattering of red, green and blue pixels have prepared full color passive matrix and active devices. In this case, each of the pixels should be efficient and should be stable to make a high operational stable full color display. White light producing OLED devices are highly desirable in the industry and are considered as a low cost alternative for several applications such as full color displays, paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. Using the white color emitting OLED and the use of color filters have also demonstrated the full color displays.
It is expected that for the display applications, the operational half-lifetime to be greater five years. In addition to the continued need to provide OLEDs having improved lifetime, it is desirable to have OLED devices with good operational performance in luminance and voltage stability over the lifetime of the OLED device under varying operating conditions. For practical applications, OLED devices should have high luminance stability and voltage stability under ambient conditions as well as higher operating temperature conditions
An OLED is actually a current driven device. Its luminance is proportional to current density, but its lifetime is inversely proportional to current density. In order to achieve high brightness, an OLED has to be operated at a relatively high current density, but this will result in a short lifetime. Thus, it is critical to improve the luminance efficiency of an OLED while operating at the lowest possible current density comprising with the intended luminance requirement to increase the operational lifetime. Much work has been done in improving the lifetime and the luminance efficiency of the OLED devices.
The following patents and publications disclose the preparation of OLEDs with improved operational lifetime. Modifications of the multilayer structure, stabilized cathode materials, and confinement of various carriers and their recombination within the emission zone have achieved significant improvement in the operational lifetime of these devices. So et al., in U.S. Pat. No. 5,853,905, discussed an EL device comprising of a single organic emission layer containing a mixture of electron-transport and hole-transport materials, sandwiched between anode and cathode. However, this device has low efficiency. Popovic et al., in SPIE Conference Proceedings, Vol. 3476, pp. 68-72, 1998, described an EL device with improved efficiency and operational lifetime prepared by mixing an emitting electron-transport material and a hole-transport material in a light-emitting layer. Xie et al., in U.S. Pat. No. 5,989,737, disclosed an OLED in which the hole-transport layer comprises a tertiary aromatic amine doped with a polycyclic aromatic hydrocarbon such as rubrene. It is also important that all the emitters should have high stability. The red, green, yellow and white emitting devices have been shown to have high operational stability. However, the blue emitting devices do not have good stability. For full color pixilated RGB display, a true blue color with excellent operational stability is required for eliminated the differential aging of the different colors in the full color panel. Substantial improvements in blue OLED luminance yield and external efficiency directly translate into lower power-consumption devices. In full-color OLED displays, the blue current density required to produce an image is directly related to the external efficiency of the blue color. Higher blue external efficiency will dramatically reduce the pixel current density required to produce an image at a given brightness, thereby increasing lifetime. Operational stability of the blue emissive materials is typically the limiting factor for overall display lifetime; therefore improvements in blue materials stability dramatically increase the overall display performance.
In OLED devices, there is a further need for improved lifetime. Commonly assigned U.S. Pat. No. 6,692,846 B2 and U.S. Pat. No. 6,565,996 B2 both show effective ways for improving such stability. However, these patents do not disclose a way to improve luminance efficiency of the light-emitting layer.